1. Field of the Invention
The present invention relates to a driving force control method in a 4-wheel drive vehicle in which the driving force of an engine is transmitted directly to one of the front and rear wheels (that is, the main driving wheels) and through driving force varying means to the other of the front and rear wheels (that is, the auxiliary driving wheels).
2. Description of the Related Art
A driving force control method in a 4-wheel drive vehicle, in which a portion of a driving force for front wheels driven by an engine is distributed to left and right rear wheels at a predetermined ratio, is conventionally known and has already been proposed in Japanese Patent Application No. 7-295762 by the present applicant.
In the above known driving force control method, if the front wheel speed is higher than the rear wheel speed as a result of slipping of the front wheel at the starting of the vehicle or during sudden acceleration of the vehicle, a portion of the driving force for the front wheel is distributed to the left and right rear wheels to bring the vehicle into its 4-wheel drive state. If a speed difference is generated between left and right rear wheel speeds by turning of the vehicle, it is ensured that the amount of driving force distributed to outer wheels during turning of the vehicle is larger than the amount of driving force distributed to inner wheels during turning of the vehicle, thereby providing an enhancement in turning performance.
If a larger amount of driving force is distributed to the rear wheels when such vehicle is being turned in its 4-wheel drive state, the travel resistance is increased and hence, there is a possibility that problems occur such as a deterioration of specific fuel consumption, an abnormal sound generated in the drive system, an increase in load of the drive system and the like.